Journal of Atrial Fibrillation – Journal of Atrial Fibrillation & Electrophysiology (JAFIB-EP)

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These are curated opinions and are not peer-reviewed.


Kimberly D. O’Donogue

Content Contributor

Questions : What is observed in the second electrogram?



  • Both examples demonstrate extrastimulus testing in the ventricle via RV pacing. Extrastimulus testing is a pacing technique in which there is a drive train of eight beats (S1) followed by an extrastimulus (S2). 
  • Familiarize yourself with the electrogram before interpreting. First, is four ECG leads, followed by the HRA channels, RV, His, and then the CS. Here a duodecapolar catheter was used for the coronary sinus catheter.   
  • Looking at the first electrogram, the last two beats of the drive train are displayed followed by the S2.  This is ventricular pacing as evident by the wide QRS complex immediately after the pacing artifact (pacing artifact seen throughout the channels). Looking at the drive train, each impulse captured the ventricle and conducted to the atrium. Next, look at the S2 or extrastimulus (PVC). The S2 captured the ventricular tissue and conducted to the atrium with delayed conduction.  
  • Also of note, there is only a small far-field A wave on the His catheter due to catheter positioning.  
  • Now, begin interpreting the second electrogram with extrastimulus of 600/290, the PVC (S2) was brought in 10 ms sooner.
  • The last beat of the eight-beat drive train was displayed. This S1 impulse demonstrates ventricular capture with a wide complex QRS immediately following the pacing artifact. The S1 did conduct to the atrium.
  • The S2 displays the pacing artifact, but there is no ventricular capture. There is a pacing artifact seen on the T wave of the previous beat, but it is not associated with a QRS complex. The impulse came in too early when the ventricular tissue was still refractory. This is called the ventricular effective refractory period or VERP.